This invention relates generally to an improved stage of an axial flow steam turbine, and more particularly to improvements in the last stage of an axial flow steam turbine for increasing efficiency thereof, thereby increasing overall turbine efficiency.
A stage of a steam turbine typically comprises a diaphragm including a plurality or set of circumferentially aligned and spaced apart stationary nozzle partitions and a plurality or set of circumferentially aligned and spaced apart rotating blades or buckets, fixedly secured to a turbine rotor at a predetermined axial position along the rotor and operatively spaced downstream from the corresponding plurality of nozzle partitions of the stage. Nozzle partitions of one stage are oriented to direct steam exiting from the next preceeding upstream stage onto the corresponding plurality of buckets associated with the one stage. The terms "upstream" and "downstream" are used herein with respect to the general axial flow of steam through the turbine.
Basically, energy is imparted to the rotor and bucket assembly of a steam turbine by an elastic working fluid, commonly steam. Steam is vented through a set of nozzle partitions of a diaphragm into a generally cylindrical chamber defined by the inner shell of the turbine housing. The shaft or rotor is coaxially and rotatably mounted within the chamber. Large steam turbines usually include several stages, each stage axially spaced apart from adjacent stages on the rotor shaft and stages sequentially increasing in diameter from the first or most upstream stage, near the point of admission of steam to the turbine, to the last or most downstream stage of the turbine which is proximate the exhaust conduit or hood of the turbine. From the exhaust conduit or hood of a low pressure turbine, spent steam is ultimately conveyed to a condenser. Generally, the ratio of the input pressure to the output pressure of rotor buckets of the last stage is greatest with respect to buckets from all other stages of the turbine, respectively.
Steam is admitted through the set of nozzle partitions of a stage into the chamber at a desired axial location and flows at least in one axial direction through a working passage. In a double flow turbine steam is centrally admitted and flows in generally opposing axial directions toward respective last stages. The working passage is generally defined by the axially displaced stages of the turbine as well as by the circumferential working area encompassed by the aerodynamic section (commonly called blade or vane profile) of turbine buckets in each stage. Each set of buckets extracts a part of energy available from steam by changing a portion of the available fluid kinetic energy into mechanical energy, as evidenced by operational rotation of the shaft and associated buckets of the turbine.
When steam is confined to the axial working passage, the turbine operates more efficiently than if steam is not so confined. Present twenty-six inch last stage buckets for a low pressure steam turbine manufactured by the General Electric Company are interconnected by tie wires and do not include covers connecting the outer tip portions of the buckets. A cover or cover piece has been used to connect together the outer tip portions of a pair of buckets from a last stage having longer buckets, say 30 inch and 33.5 inch. A plurality of covers, which correspond to the plurality of rotor buckets in the turbine stage, form a circumferential band around the radially extensive tip portions of the buckets. This circumferential band of covers prevents some steam from escaping from the axial working passage by limiting radial flow of steam past the outer tip portions of the buckets. The rotor and bucket assembly must be free to rotate within the turbine shell and therefore, a radial clearance gap exists between the radially extensive tips of the rotor buckets or outer surface of the covers, and the inner surface of the shell of the turbine.
In the last stage of a low pressure steam turbine working steam is normally below the saturation line. Therefore water droplets are apt to form upstream of last stage buckets, such as in the region of the last stage nozzle and diaphragm. Generally, water droplets are forced radially outward from the shaft by centrifugal force. Although water droplets generally have a low absolute velocity, the relative velocity, especially with respect to radially outer portions of last stage buckets is very fast, about equal to bucket tip tangential velocity.
Water droplets impinging leading edges of last stage buckets may cause impact erosion of the edges. Most erosion damage results from condensed moisture of preceeding stages which forms a film of water over last stage nozzle partitions. The film of water is continuously sheared off to form particles of water at trailing edges of last stage nozzle partitions by high velocity steam which sweeps over the partitions. Water particles move such a short distance between trailing edges of nozzle partitions until potential contact with a leading edge of a bucket that they cannot be accelerated to a very high absolute velocity and thus appear as relatively stationary objects with respect to rotating buckets.
The relative velocity of water droplets near bucket tips in a low pressure turbine which includes a last stage, active bucket length of about 26 inches is approximately fifteen hundred-fifty feet per second. The force at which a water droplet impacts a bucket blade is related to size or mass of the impinging droplet and relative velocity of the droplet with respect to the bucket. Since speed of the turbine is essentially established by other parameters, potential problems caused by water droplets, such as erosion, lower torque, and loss of efficiency, can be minimized by providing a turbine rotor and bucket assembly which effectively limits the amount of water and number and size of water droplets in the axial working passage of the turbine.
As stated earlier, the pressure ratio across the last stage of the turbine is greatest as compared with other upstream stages of the turbine. Also, the pressure differential across last stage buckets is generally higher near the radially outer portion of the rotating blades as compared with the root or radially inner portion of the blades. Therefore, the greater the radial clearance gap between the radially outermost rotatable component of the last stage and the inner surface of the shell, the greater the loss of steam and hence, the lower the efficiency of the last stage of the turbine.
It is important to insure that maximum working steam be forced through last stage buckets in order to extract available energy therefrom and that working steam which bypasses the last stage buckets be minimized. To minimize loss of steam flow around the outer portions of buckets, sealing strips have been placed on the inner surface of the turbine shell radially opposite the tip portions and covers of buckets in prior art apparatus. Generally, the sealing strips form a ring around the buckets and extend radially inward towards the bucket tip portions to narrow the radial clearance gap therebetween. The number of strips utilized per stage and the axial placement of the strips on the inner surface of the shell is based upon a study of fluid mechanics in a steam turbine. Sealing strips should be axially located such that the strips are approximately opposite the steady state centerline of the rotating buckets.
The steady state centerline is the centerline of buckets when the turbine is in normal operation at rated speed. However, since the rotor shaft, upon which the buckets are mounted, expands due to thermal reaction to steam, optimum axial placement of sealing strip, i.e., at the steady state centerline, is not easily ascertained. Also, the axial position of rotating blades changes during operation of the turbine, especially when the turbine experiences transient changes in its mechanical load or changes in the condition and volume of steam supplied thereto.
Prior attempts to prevent steam from escaping and bypassing the working passage of the last stage have also included common labyrinth seals disposed in the radial gap between the radially outermost portion of the bucket cover and the inner surface of the shell. Labyrinth seals typically comprise ribs radially extending from the bucket cover which cooperate with circumferential flanges inwardly projecting from the inner surface of the shell. Projections from the inner surface of the shell prevent water from smoothly flowing past the last stage buckets along the inner surface of the shell and may cause water droplets to fall into the working passage of the last stage from the projections. When labyrinth seals are used, a moisture removal channel disposed through the inner wall of the shell immediately upstream the seal permits a portion of the working steam to escape through the channel, thus carrying water along with it. A similar moisture removal channel is required if the aforementioned sealing strips are used.
Although steam leakage flow around the outer tip portions of buckets is reduced by incorporation of labyrinth seals, some working steam is lost through the moisture removal channel without having passed through the last stage buckets. Further, steam and water exiting through the moisture removal channel is at a higher pressure than the input pressure to the condenser from the output of the last stage and thus appropriate conduits and orifices may be necessary for connecting the moisture removal channel to the condenser in order to adjust the pressure of steam and water from the water removal channel to minimize flow of leakage steam to the condenser.
The design of the last stage of a steam turbine to achieve optimal operating efficiency requires use of interdisciplinary science and engineering such as aerodynamic, structural, mechanical and manufacturing along with generally several iterations of design alternatives. It is especially worthwhile to ensure that operation of the last stage yields optimum stage efficiency since the last stage recovers substantially more energy, typically about 10% of the overall turbine output, from steam than any other stage in the turbine and thus has a significant impact on overall efficiency of the turbine. Other factors which make design and operation of a last stage different from other stages of a turbine include: higher volume flow of steam through the last stage than through any other stage and therefore, last stage buckets are longest and subject to highest stresses; ability to efficiently operate with variable exhaust pressure (upstream stage outputs are at relatively constant pressure ratio) resulting in variable stage pressure ratio, variable energy output, and variable aerodynamic conditions; greater moisture content in last stage working steam than any other stage; and, last stage buckets have highest tip speed, highest flow velocities and greatest three-dimensional flow effects with respect to buckets of any stage in the turbine.
Last stage buckets of low pressure turbines, i.e., turbines having a steam output design pressure from the last stage typically less than about 5.0 inches of mercury absolute, generally have a long and thin bucket profile, and are thus subject to untwisting due to centrifugal forces acting thereon during turbine operation. It is desirable that the untwist be accounted for so that turbine buckets obtain optimum aerodynamic relationship during normal turbine operation. At nominal 3600 rpm operational speed the speed of the bucket in the tip section may be about 1550 feet per second for a 26 inch last stage bucket which creates a relative supersonic environment for steam flowing between turbine blades. It is important to control the distribution of the transition region from subsonic to supersonic flow through last stage buckets in order to prevent undesirable shock waves and corresponding loss in efficiency. In addition, it is possible to obtain supersonic steam flow through last stage nozzle partitions and likewise the transition region from subsonic to supersonic flow must be controlled to ensure that desired steam flow conditions are maintained through the nozzle partitions to the input at the last stage buckets. An improper or unexpected transition region through nozzle partitions may result in a loss of efficiency due to undesirable shock patterns. A transition from subsonic to supersonic flow may be accompanied by a shock wave which causes an irreversible loss of pressure, i.e., pressure is lost and cannot be recovered to produce mechanical energy.
In contrast to the last stage of a low-pressure steam turbines, gas turbines generally employ integral covers over bucket tips which prevent untwisting of buckets; gas turbine bucket profiles are generally short and stubby and typically are manufactured from a superalloy with a coating to resist the harsh gas turbine environment; gas turbine last stage discharge pressure is relatively constant i.e., atmospheric; and gas flow through a gas turbine is an open system whereas steam flow through a steam turbine, and subsequent steam condensation and water reheat to form steam, is a closed system. Although steam turbines may experience problems with occluded water or condensed steam as hereinbefore mentioned, the harsh environment of a gas turbine generally does not exist within a steam turbine and thus, in view of the foregoing, it would generally not be expected that one skilled in the art of steam turbine design and manufacture would look to gas turbine art to teach or suggest solutions which may be specifically applicable to steam turbines.
Accordingly, it is an object of the present invention to provide a sealing arrangement for retaining steam within the axial working passage of a stage of an axial flow steam turbine while protecting stage components from mechanical damage due to moisture without prematurely removing moisture from the stage.
Another object is to provide positive control over the positioning of the elastic fluid flow transition region from subsonic to supersonic (i.e. transonic expansion region) in the last stage of a low pressure steam turbine to prevent formation of undesirable sonic shocks during operation.
Yet another object is to control untwist of last stage steam turbine buckets to obtain optimum aerodynamic orientation during normal operating conditions.
Still another object is to provide optimum diaphragm and bucket cooperation to supply desired steam flow and to help delay onset of recirculating flow as manifested by bucket root flow separation at low average annulus velocity of elastic fluid flow through the last stage of a steam turbine.